"Fact is, when you're looking at photos from last night's holiday party on Facebook, you're probably ignoring that teeth-whitening ad. And with all the user-generated content, these sites have so many page views that Web 2.0 companies can't command the same rates as, say, portals. Yahoo's news site, for example, can charge more than 30 times as much as Facebook for a banner ad."
I think that as long as advertising is the sole way to make money on the web, social media will struggle to be profitable. There should be a more direct way to monetize the value that people get out of their social networking tools.